Interpretive Summary: The rapid and widespread adoption of glyphosate-resistant crops, associated with the intense use of glyphosate and lack of rotation with non-glyphosate-resistant crops has resulted in evolution of several glyphosate-resistant weed species, including tall waterhemp in Mississippi and several other states in the US. Glyphosate-resistant tall waterhemp populations could reduce yield and quality of row crops such as corn, cotton, rice, and soybean. Scientists from the Crop Production Systems Research Unit (USDA-ARS), other ARS research units, and Mississippi State University conducted studies to verify resistance to glyphosate, to explore alternative chemical control methods, and to understand why tall waterhemp in Mississippi is resistant to glyphosate. A tall waterhemp population in a glyphosate-resistant soybean field, Washington County, Missisippi, was 5-fold more resistant to glyphosate than a sensitive population. Among several non-glyphosate herbicide treatments evaluated for efficacy on the glyphosate-resistant tall waterhemp population, only 2,4-D and mesotrione proved to be effective. The glyphosate-resistant tall waterhemp population absorbed and translocated less glyphosate compared to a susceptible population. Further, a change in the DNA of an enzyme on which glyphosate is active was detected in the glyphosate-resistant population, thereby, making glyphosate ineffective. These results indicate a uniqueness of two modes of resistance to glyphosate in the tall waterhemp population, complicating management decisions.

Technical Abstract:
A tall waterhemp population in a glyphosate-resistant soybean field, Washington County, Missisippi, was suspected to be resistant to glyphosate. Glyphosate dose response experiments resulted in GR50 (glyphosate dose required to cause a 50% reduction in growth of treated plants) values of 1.28 and 0.28 kg ae ha-1 glyphosate for the glyphosate-resistant (GR) and -susceptible (GS) populations, respectively. This indicated that the GR population was 5-fold more resistant to glyphosate than the GS population. Among several non-glyphosate herbicide chemistries evaluated for efficacy on the GR population, only 2,4-D and mesotrione provided more than 90% control 3 wk after treatment. The absorption pattern of 14C-glyphosate between the GR and GS populations was similar up to 24 h after treatment (HAT). Thereafter, the susceptible population absorbed more glyphosate (55 and 49% of absorbed) compared to the resistant population (41 and 40% of absorbed) by 48 and 72 HAT. Treating a single leaf with glyphosate solution at the field use rate (0.84 kg ha-1) as ten 1-µl droplets provided greater control (85% vs. 29%) and shoot fresh weight reduction (73% vs. 34% of nontreated control) of the GS plants compared to the GR plants, indicating a reduced movement of glyphosate. The amount of 14C-glyphosate that translocated out of the treated leaves of GR plants (19.8% of absorbed at 24 HAT and 23.3% of absorbed at 48 HAT) was significantly lower than the GS plants (30.9% of absorbed at 24 HAT and 31.9% of absorbed at 48 HAT). Phosphorimaging results further corroborated the above results.